In early 1999, Karen Cassidy, a dental hygienist and former high school athletics official, saw ads touting the new LYMErix vaccine for preventing Lyme disease, a bacterial infection passed to people by the bloodsucking bite of the deer tick. “Deer are right at my back door,” says Cassidy, who hoped the vaccine would let her rake the yard of her suburban Philadelphia home without fear. But shortly after she completed two of a course of three inoculations in May 1999, Cassidy began experiencing burning pain in her back, numbness in her arms and aches and swelling in an ankle. “Just the thought of walking across the yard hurt me,” says Cassidy.

By the following December, the pain was so intense she was considering reconstructive surgery for her ankle and had joined over 100 others in a class action lawsuit charging LYMErix’s developer, the pharmaceutical giant SmithKline Beecham, with, among other things, ignoring warnings that a genetically identifiable class of recipients-as many as 30 percent of patients-may develop an incurable autoimmune disorder called treatment-resistant Lyme arthritis from the vaccine.

While Cassidy’s diagnosis is in dispute, her legal case is an undeniable indicator of how dramatically medicine is being transformed by a new science known as pharmacogenomics.

Now that the Human Genome Project is largely complete, academics and companies are shifting their emphasis to the study of how genes vary among people. The Human Genome Project involves decoding the sequence of one complete set of human genes, a kind of genetic Everyman. But that template won’t describe everyone, since DNA varies slightly from person to person. And those minute variations in the DNA, scientists believe, may determine which patients will benefit most from particular drugs-as well as which subgroups may be harmed by them.

That’s the idea of pharmacogenomics, and it is taking the drug industry by storm. The term, coined only four years ago, now encompasses the aspirations of a large number of enterprising biotechnology companies and academic laboratories. This is an era of transition in medicine: from the time of “one size fits all” drugs created for and marketed to all patients, to the emerging epoch of personalized medicines, in which drugs are geared to the specific genetic makeup of groups or individuals. This transition is causing growing pains for some companies. But the ultimate payoff, a decade or more away, should be enormously beneficial to patients, enabling doctors to think about stopping tumors before they begin and heart attacks before they happen.

“This is not a fad,” says Gualberto Ruano, CEO of Genaissance Pharmaceuticals in New Haven, Conn., a player in the transition. “It’s a major tidal wave changing the entire pattern of health care.” And Alan Roses, who leads genetics research at drug giant Glaxo Wellcome in Research Triangle Park, N.C., agrees wholeheartedly that pharmacogenomics “is a disruptive technology, not a technology that sustains what organizations are used to doing. It’s going to be a part of everybody’s business, and that’s what most people don’t seem to understand.”